English: Illustration of experiment demonstrating wireless power transmission by Nikola Tesla, in 1921 do-it-yourself magazine. The circuit functions as a Tesla coil. The transmitter circuit (left) is a spark-excited resonant oscillator. The capacitor (C) and primary coil (P) form a resonant circuit. The capacitor repeatedly charges up, then discharges through the spark gap (S-G), creating radio frequency (RF) oscillating currents in the primary. These induce alternating currents by Ampere's law in the secondary coil (S), powering the lamp. The frequency of the oscillating currents was around 100 kHz to 1 MHz. Tesla used similar circuits in public lectures during the 1890s to light lamps wirelessly at distances of tens of feet.